1. Field of the Invention
The present invention relates to memory elements. Specifically, the present invention relates to a memory element suitable for pixel driving of an active-matrix display device. Furthermore, the present invention relates to an active-matrix display device for which such a memory element is formed in each pixel.
2. Description of Related Art
An active-matrix liquid crystal display device includes gate lines on rows, data lines on columns, and pixels disposed at the intersections of the gate lines and the data lines. In each pixel, an electro-optical element typified by a liquid crystal cell and an active element, such as a thin film transistor, for driving the electro-optical element are formed. The gate of the thin film transistor is connected to the gate line, the source thereof is connected to the data line, and the drain thereof is connected to the electro-optical element. The active-matrix display device line-sequentially scans the gate lines and supplies video signals (data) to the column data lines in linkage with the gate line scanning, to thereby display an image dependent upon the video signals on a pixel array.
Specifically, the active-matrix display device line-sequentially scans the gate lines every one field and supplies the video signals to the data lines in linkage with this scanning. In the case of displaying of a moving image, the picture on the screen is switched every one field, and therefore charging and discharging of the video signals in the data lines needs to be repeated every one field. In the driving of the panel of the active-matrix display device, most of the power consumption is due to the charging and discharging of the data lines.
To suppress the power consumption due to the charging and discharging, it is effective to decrease the frequency of the image rewriting (field frequency). However, it is well known that decreasing the field frequency to a value in the range of 30 to 60 Hz or lower causes flicker on the screen and thus deteriorates the display characteristics. To address this problem, as a related-art scheme for saving the power consumption without decreasing the field frequency, a system has been proposed in which a memory function is incorporated in each pixel to thereby decrease the number of times of charging and discharging. For example, this system is disclosed in Japanese Patent Laid-open No. Hei 11-52416 and M. Senda et al. “Ultra low power polysilicon AMLCD with full integration” SID2002, p 790.
Studies have been advanced for a technique to decrease the number of times of charging and discharging of the data lines and thereby reduce the power consumption by continuing displaying of the data held by the memory function in the pixel when the input video signal does not change, such as when a still image is being displayed.
For example, there has been proposed a system in which an SRAM memory element is integrated in each pixel in order to incorporate the memory function in the pixels in a liquid crystal panel. However, for the SRAM memory element, at least six transistors are used per one bit. Therefore, if six bits are assigned to each one pixel and thus 64-grayscale displaying is intended, 6×6=36 transistors need to be integrated per one pixel, which correspondingly puts pressure on the effective aperture area of the pixels. Because the area of the pixel aperture that allows the passage of a light beam from a backlight necessary for displaying is decreased, a bright screen cannot be obtained. Thus, in the case of incorporating a related-art memory element in the pixels as it is, increase in the number of bits is difficult, which imposes the limit to high-definition multi-grayscale displaying; this problem should be solved.
The above-mentioned Japanese Patent Laid-open No. Hei 11-52416 discloses an example in which ferroelectric is used as a system for realizing the memory function incorporated in the pixels. In this system, there is no fear of putting pressure on the aperture area because a circuit element such as a transistor does not need to be formed in each pixel. However, a material proper for the ferroelectric having the memory function is very few, and thus this system has not yet reached the practical-use level. Specifically, it is said that the ferroelectric characteristics and the insulation properties tend to be easily changed through repetition of data rewriting and therefore it is difficult to ensure the reliability of the memory function.